Among the battery which has been practically used, the lithium ion secondary battery exhibits the highest energy density, and particularly, it is widely used for the compact electronics. Also, it is expected to be used in the automobile in addition to the compact use. Under such condition, the lithium ion secondary battery is demanded to have further long lifetime, and further improved safety.
The lithium ion secondary battery is provided with positive electrodes and negative electrodes including the active material layer (hereinbelow, it may be referred as “the electrode active material layer”) generally supported at current collector, a separator, and a nonelectrolyte solution. The active material layer includes electrode active materials and a binder. The electrode is produced by forming the active material layer by coating the slurry mixture including the electrode active material in a powder form onto the current collector. Also, as the separator which separates the positive electrode and the negative electrode, a very thin separator having the thickness of 10 to 50 μm or so is used. Particularly, as the separator of the lithium ion secondary battery; an organic separator of a polyolefin such as polyethylene, polypropylene or so may be used. The polyolefin organic separator has a physical property which melts at 200° C. or less. Thus in case the battery becomes very high temperature unexpectedly due to the stimulus form inside or outside, the volume change such as shrinking or melting may take place, and as a result, the short circuit at the positive electrode and the negative electrode may take place. Also, the lithium ion secondary battery is produced by going through the stacking step of the electrode and the separator, and the cutting step wherein the electrode is cut into predetermined shape. However, while going through these production steps, the active material is released from the active material layer, and a part of the released active material may be included in the battery as the foreign matter.
In order to prevent the short circuit caused by such unexpected high temperature or by the foreign matter, it is proposed to stack a layer (the porous membrane) comprising an inorganic particle or non-conductive particle such as resin particle having high heat resistance on the electrode (the positive electrode or the negative electrode) or on the polyolefin organic separator.
For example, in the patent article 1 (JP Patent Application Laid Open No. 2005-327680), the porous membrane is formed by coating the paste including the rubber particle having acrylonitrile group as the binder, and the alumina particle which is an non-conductive inorganic oxides as the filler, to the electrode. By providing such porous membrane, the active material is prevented from being released during the production steps of the battery, and thus the short circuit is prevented from occurring while the battery is operating. Further, since the porous membrane is porous, the electrolytic solution permeates into the porous membrane, thus the battery reaction is not interfered.